The present invention relates to a vehicle speed control apparatus of an automobile and more particularly relates to an apparatus for automatically maintaining the vehicle speed at a desired speed without driver operation of the accelerator pedal. Such operation is frequently referred to as cruise control.
A conventional vehicle speed control apparatus consists of a storing device which stores the actual running speed of an automobile at the time when a set switch is operated, a comparator which compares the actual running speed with the stored running speed, and a speed adjustor which adjusts the opening degree of a throttle valve to maintain the running speed of the automobile at the stored running speed, in other words, at the desired running speed.
In such a vechicle speed control apparatus, after the set switch has been actuated by the driver for setting the desired vehicle speed to be maintained (setting operation), the set switch, is additionally used for decreasing the desired vehicle speed which has been set (referred to as the "retard operation") as well as used for slightly increasing the desired vehicle speed. The set switch is actuated and then released by the driver, and the setting operation is effected by storing an electrical signal representing the actual vehicle speed when the operating set switch returns to the non-operating condition. The speed thus selected is referred to as the desired vehicle speed. The retard operation is effected by moving the throttle valve toward its closing direction when the set switch is operating. The tap-up operation is effected by briefly actuating the set switch when the vehicle is operating under control of the speed-controlling apparatus, thereby slightly increasing the stored vehicle speed, for example, by increasing the stored vehicle speed by 2 to 3 km/h when the operating set switch returns to the non-operating condition. Furthermore, in such a vehicle speed control apparatus, a high-speed limit circuit is provided for forcibly inhibiting all the above-mentioned functions of the set switch from being done when the actual vehicle speed exceeds a predetermined upper limit speed, for example, a speed of 100 km/h.
However, conventional vehicle speed control systems of the above-mentioned type have the following problem. If the stored vehicle speed signal exceeds the predetermined upper limit speed due to carrying out the tap-up operation and thus the actual vehicle speed exceeds the upper limit speed, it becomes quite impossible to decrease the vehicle speed by operating the set switch, in other words, quite impossible to excecute the retard operation. This is because all functions of the set switch are inhibited by the high-speed limit circuit when the vehicle speed exceeds the predetermined upper limit speed.